1. Field of the Invention
The invention relates to a method for the preparation of silicon oxide films on silicon surfaces with at least two oxidation steps, in which a first step is performed at a low oxidation temperature in an atmosphere containing a mixture of oxygen and hydrogen chloride with a low hydrogen chloride concentration, and in which a second step is carried out at a high oxidation temperature.
2. Description of the Prior Art
In the fabrication of large-scale integrated circuits in MOS technology, small oxide thicknesses of high quality are required for the gate oxides particularly where the oxide layers applied to a semiconductor surface should have above all a uniform and reproducible thickness, a high breakdown voltage, a low defect density, small surface charges and good passivation.
High oxide quality can be obtained by very elaborate purification processes; otherwise, it is known to perform the oxidation of silicon surfaces in a gas mixture of oxygen and hydrogen chloride. In the latter method, however, low oxidation temperatures result in low defect rates and poor passivation, and high oxidation temperatures in high defect rates and good passivation.
The quality of thin silicon oxide films may be improved by a two-step oxidation process of the type mentioned at the outset described in the article "A Method of Forming Thin and Highly Reliable Gate Oxides" by C. Hashimoto, S. Muramoto, N. Shiono and O. Nakajima, Journal of the Electrochemical Society, January 1980, pages 129-135. In this process one works in the first oxidation step with an oxygen/hydrogen chloride mixture with a relatively low hydrogen chloride concentration and relatively low temperature, and in the second oxidation step with a relatively high temperature and a gas mixture of nitrogen, oxygen and hydrogen chloride. Mixing these three gases in the second oxidation step, however, is a disadvantage particularly because the nitrogen content of the three gases is more than 90% by volume, and since the gas components do not mix homogeneously, the individual components are not distributed simultaneously on the semiconductor wafer. This leads, particularly at the boundary surface between silicon and silicon dioxide, to silicon nitrite layers which degrade the long-term behavior of the oxide layers. The high oxidation temperatures of 900.degree. to 1100.degree. C. in the first step and above 1100.degree. C. in the second step further degrade, through unavoidable diffusion processes, the semiconductor properties of semiconductor wafers already doped prior to the oxidation. Lower oxidation temperatures, however lead with the mentioned gas mixture to long oxidation times.